motto vs underword

motto

noun
  • A sentence, phrase, or word, prefixed to an essay, discourse, chapter, canto, or the like, suggestive of its subject matter; a short, suggestive expression of a guiding principle; a maxim. 

  • A sentence, phrase, or word, forming part of an heraldic achievement. 

underword

noun
  • A word placed below another, either literally or metaphorically: a subtext; an inadequate or subordinate term or description; a secret word or phrase; a watchword 

How often have the words motto and underword occurred in a corpus of books? (source: Google Ngram Viewer )